This invention relates generally to a new and improved joint prosthesis, and more particularly relates to an improved multi-component joint prosthesis, such as for example a multi-component hip joint prosthesis, wherein at least one of the components may be an open shell.
The conventional prior art total hip prosthesis typically includes a metal femoral head fixtured to the femur and a plastic, generally ultra-high molecular weight (UHMWPe), cup fixtured by cement to the acetabulum such as the type disclosed in U.S. Pat. No. 4,123,806 issued Nov. 7, 1978 to Amstutz et al. The prior apt also discloses the use of hip prostheses including metal backed acetabular components in which the acetabular component consists of a metal cup with a plastic bearing insert; examples of such prostheses are given in U.S. Pat. No. 3,840,904 issued Oct. 15, 1979 to Tronzo and U.S. Pat. No. 3,903,549 issued Sep. 9, 1975 to Deyerle. Metal backed acetabular hip prosthesis components have several advantages. The metal or rigid outer acetabular cup produces a much more uniform stress distribution at the interface between the cup and the acetabulum with lower peak forces or stress thereby improving the possibility of long term fixation. Rigidity of the metal outer acetabular cup also reduces distortion of the plastic liner improving its sphericity and therefore contact with the metal femoral head of the hip prosthesis, thereby improving conditions for wear resistance. Further, the use of a separate bearing insert allows replacement of the insert if the insert is damaged intraoperatively, or if the insert becomes excessively worn as a result of long term use, or if as a result of some problem revision is necessary which involves a change in the insert bearing. For example, in the event of revision from a surface replacement type hip prosthesis, which has a fairly large diameter head, to a stem type femoral prosthesis, which includes usually a smaller diameter head, one may simply remove the bearing liner leaving the metal acetabular cup affixed to bone, and replace the liner with another liner or insert of appropriate size for the revised unit. Such revision can, therefore, be made without disturbing the acetabular fixation thereby preventing damage to the acetabular bone.
There is, however, a potential disadvantage associated with the use of a rigid or metal acetabular cup. In the event that a load is applied near the rim of the acetabular cup, the elastic properties of the underlying bone combined with the rigidity of the cup can produce a situation in which the opposite rim tends to be lifted off of its bony bed. This tends to produce tensile loads on the cup and such tensile loads are undesirable in maintaining long term fixation.
In the prosthesis disclosed in the Tronzo patent, an interlock 26 and interlocking groove 28 as described in FIGS. 2-8 of Tronzo are used to prevent rotation of the liner relative to the cup. This connection, however, is not resistant to a tensile withdrawal of the cup from the liner. Such withdrawal can occur as a result of traction forces due to a layer of liquid interposed between the femoral component and the plastic liner coupled with distraction of the femur from the acetabulum during normal activity. Such a situation can produce withdrawal of the plastic bearing from the acetabular cup producing dislocation of the prosthetic component.
The prosthesis described in the Deyerle patent uses an arcuate ridge 32 which engages an arcuate slot 44 as shown in FIGS. 1, 5 and 6 of Deyerle to restrict relative rotation. The Deyerle device uses screws by making use of an annular liner (not identified by number) in conjunction with a retaining screw 30 to trap the liner in the cup. This design, however, experiences difficulty in liner removal because removal of the liner requires removal of the screws from the bone which may produce damage to the bone. Further, the use of such a connection resistant against tensile loading requires the use of screws and many surgeons would prefer in some applications not to use screws for fixation.
Both the Tronzo and Deyerle prostheses use screws. Neither, however, provides the ability of the screws to change their angular orientation significantly to facilitate fixation. Further, neither provides screws near the inferior rim to minimize the possible lifting of the inferior rim as a result of loads applied near the superior rim. Such loads are normal in walking and may exceed eight times body weight in stair climbing and descent. Since the angular orientation of the screw is not adjustable in Tronzo and Deyerle devices, these screw configurations cannot take maximum advantage of possible superior bone stock for screw implantation. Further, Deyerle and Tronzo prostheses both make use of either screws or spikes for fixation. However, when such acetabular components are used with cement, such spikes or screws may not be necessary for fixation and their use makes the operative procedure more difficult and introduces additional damage to the bone.
U.S. Pat. No. 3,608,096 to Link discloses a prosthesis which uses a relieved face on the acetabular shell where a segment of the shell is removed by means of an oblique cut (not identified by number) as described in Column 2, lines 69-72 and Column 3, lines 1-3 to provide a better approximation to the shape of the natural acetabulum so as to increase clearance reducing possible impingement with the femur during certain kinds of activity as described in Column 3, lines 26-29. Further, the outside section 3 as shown in FIG. 1 of the Link patent is eccentric to the cavity 2 although the nature and reason for this eccentricity are not described by Link. Although this oblique and simple relief provides improvement in fit and clearance, still better fit and clearance can be provided by a somewhat more complex relief of the inferior face of the acetabular component.
In view of the above, it is an object of the subject invention to provide a multi-component prosthesis that facilitates assembly and disassembly of the components.
A further object of the subject invention is to provide a multi-component prosthesis that enables the components to be securely retained in their assembled condition and also achieves easy assembly and disassembly for all dimensional ranges of the components dictated by manufacturing tolerances and environmental conditions.
Another object of the subject invention is to provide multi-component prostheses that provide a smaller range of insertion forces for any given range of manufacturing and other size variations.